1. Field of the Invention
The present application relates to a method to drive a Mach-Zehnder modulator (hereafter denoted as MZ-modulator), in particular, the application relates to a method to drive a semiconductor MZ-modulator.
2. Related Background Arts
Many prior arts have disclosed an MZ-modulator that provides an input optical waveguide to guide an input optical beam, a branch to divide the input optical beam into two beams, a pair of phase modulators each coupled with the branch, an optical coupler to couple two beams each divided by the optical branch and propagated in the phase modulators into a composite optical beam, and an output optical waveguide to guide the composite optical beam. These members of the input optical waveguide, the optical branch, the phase modulators, the optical coupler, and the output optical waveguide, are monolithically integrated on a substrate. Each of the phase modulators has an equivalent refractive index different from others. The phase difference between optical beams each propagating in the phase modulators are given by (2n+1)×π, where n is zero or positive integers, under a condition of no modulation signal. That is, two optical beams each output from the phase modulators countervail to each other under such a condition, which results in no optical output from the MZ-modulator.
As the volume to be transmitted by the optical communication system explosively increases, an additional technique fundamentally different from the conventional magnitude modulation has been requested. The optical QPSK (Quadrature Phase Shift Keying) technique is one of the solutions for such requests. A transmitter operable in the QPSK mode includes a laser diode (LD) as an optical source and an optical phase modulator to modulate the optical beam emitted from the LD by the QPSK mode. The QPSK modulator is constituted by a pair of MZ-modulators. However, when the MZ-modulator is made of semiconductor material, various subjects to be solved have been known.